The present invention relates to a storage unit using a memory element of variable resistance and a method of determining a reference resistance thereof.
In recent years, there are strong needs for storage unit for computer having performances such as high writing speed, non-limitation in number of writings and yet non-volatile, and thus attracting designers' attention as storage media having these performances is a ferromagnetic tunnel junction device which is made by laminating a fixed magnetization layer and a free magnetization layer by way of a tunnel barrier layer.
Such ferromagnetic tunnel junction device has a characteristic that the resistance of the tunnel barrier layer becomes lower than the resistance (reference resistance) of a resistance element to be referred (a reference resistance element) in case when the free magnetization layer is magnetized in the same magnetization direction as the fixed magnetization layer, while the resistance of the tunnel barrier layer increasing higher than the reference resistance in case when the free magnetization layer is magnetized in the opposite direction to the magnetization direction of the fixed magnetization layer.
In addition, by utilizing the above mentioned characteristic to exhibit different resistance of the tunnel barrier layer in response to the magnetization direction of the free magnetization layer, the ferromagnetic tunnel junction device provides two different states in the magnetization direction depending on whether the free magnetization layer is magnetized in the same magnetization direction as that of the fixed magnetization layer or magnetized in the opposite magnetization direction to that of the fixed magnetization layer and the ferromagnetic tunnel junction device is made to store data by defining such two different magnetization states as the data “0” or “1”.
As described above, the ferromagnetic tunnel junction device is acting as a memory element of variable resistance which takes a high resistance state having a higher resistance than the resistance of the reference resistance element or a low resistance state having a lower resistance than the resistance of the reference resistance element depending on the two kinds of data to be stored.
A storage unit utilizing such memory element of variable resistance as the memory medium is designed to read out the data from the memory element of variable resistance by making a judgment which of the two kinds of data is stored in the memory element of variable resistance. It is therefore necessary to make a judgment if the memory element of variable resistance is in the high resistance state or in the low resistance state.
A known storage unit to make a judgment if such memory element of variable resistance is in the high resistance state or in the low resistance state can be classified into two kinds which will be described hereunder.
In other words, in order to store a single data, a first conventional storage unit includes a pair of (or two) resistance change memory elements which constitute a main element and a sub element and are formed on a common semiconductor substrate. The main memory element of variable resistance is made to exhibit the resistance state corresponding to the data to be stored, while the sub memory element of variable resistance is made to exhibit the resistance state opposite to that of the main memory element of variable resistance. Comparison is made on resistances of these two memory elements of variable resistance to determine that the main memory element of variable resistance is in the high resistance state if the resistance of the main resistance change device is higher than that of the sub memory element of variable resistance, while the main memory element of variable resistance is in the low resistance state if the resistance of the main memory element of variable resistance is lower than that of the sub memory element of variable resistance.
In addition, a second conventional storage unit includes a single reference resistance element for a plurality of memory elements of variable resistance formed on a common semiconductor substrate. The resistance of the reference resistance element is set to the resistance between the resistance of the memory element of variable resistance in the high resistance state and the resistance in the low resistance state. It is determined that the memory element of variable resistance is in the high resistance state if the resistance of the memory element of variable resistance is higher than the resistance of the reference resistance element, while the memory element of variable resistance is in the low resistance state if the resistance of the memory element of variable resistance is lower than the resistance of the reference resistance element.
However, the first conventional memory apparatus as described hereinabove is required to form on a common semiconductor substrate two memory elements of variable resistance, i.e., the main memory element of variable resistance and the sub memory element of variable resistance for storing a single data, thereby doubling the number of memory elements of variable resistance to be formed on the semiconductor substrate, thereby making the storage unit less compact and increasing the production cost because a large number of memory elements of variable resistance must be fabricated more precisely.
In addition, in the second conventional storage unit as mentioned hereinabove, resistance values in the high resistance state and the low resistance state of the memory elements of variable resistance are most likely to change depending on the fabrication process. Moreover, since there is no significant difference between both resistance values, it was very difficult in the design stage to previously set the resistance of the reference resistance element to the resistance between the resistance values of the memory element of variable resistance in the high resistance state and in the low resistance state.
Also, since the resistance of the reference resistance element itself tends to vary depending on the fabrication process, it is possible that the resistance of the reference resistance element may not be the resistance between the resistance values of the memory elements of variable resistance in the high resistance state and in the low resistance state. In such a case, judgment of the storage state of the memory element of variable resistance may be made incorrectly, thereby making it impossible to correctly read out the stored data from the memory element of variable resistance.
It is therefore an object of the present invention to provide a storage unit of a structure using the reference resistance element in the second conventional memory apparatus as described hereinabove in order to avoid increasing the number of memory elements of variable resistance which are formed on a semiconductor substrate and yet capable of varying the resistance of the reference resistance element so that the stored data can be accurately read out of the memory element of variable resistance even if there are differences in the resistance value of the memory element of variable resistance and the reference resistance element.